Step 1: Go to Castle Volkihar.
Step 2: Go around the {left) side, until you reach the Undercroft part where you’ll meet three skeletons which are easy to take out. Enter the castle.
Step 3: Go through the only way you can, you’ll encounter a Skeever before reaching another door, open the door to find some Death Hounds and this is where Serana joins you as a second follower.
That’s it. Not sure how or why this happens and it’s reproducible (able to be done more than once if you happen to lose a second follower). I did the last Hircine totem quest with Aela, but that meant losing a follower, the problem is, you lose BOTH followers, so I went to get Mjoll (she was at my Riften house because we have an adopted daughter there, but we ‘live’ in Solitude) and then did this what I can only call, glitch again to regain the second companion.
*Disclaimer: I don’t know if this is elsewhere on the internet, I posted this from personal experience. Not sure if it works for the PlayStation 3 or PC versions, you’ll just have to try it for yourself!
